Add a method that return center of charge of the system

--- src/lib/chemistry/molecule/molecule.cc.orig	Wed Apr  3 15:44:33 2002
+++ src/lib/chemistry/molecule/molecule.cc	Fri Feb 11 11:37:29 2005
@@ -607,6 +607,27 @@
   return ret;
 }
 
+SCVector3
+Molecule::center_of_charge() const
+{
+  SCVector3 ret;
+  double C;
+
+  ret = 0.0;
+  C = 0.0;
+
+  for (int i=0; i < natom(); i++) {
+    double c = charge(i);
+    ret += c * SCVector3(r(i));
+    C += c;
+  }
+
+  ret *= 1.0/C;
+
+  return ret;
+}
+
+
 double
 Molecule::nuclear_repulsion_energy()
 {

--- src/lib/chemistry/molecule/molecule.h.orig	Sat Nov 23 04:00:23 2002
+++ src/lib/chemistry/molecule/molecule.h	Fri Feb 11 11:36:30 2005
@@ -209,6 +209,10 @@
         the center of mass for the molecule. */
     SCVector3 center_of_mass() const;
 
+    /** Returns a SCVector3 containing the cartesian coordinates of
+        the center of charge for the molecule. */
+    SCVector3 center_of_charge() const;
+
     /// Returns the nuclear repulsion energy for the molecule
     double nuclear_repulsion_energy();